Learning to cope with biting flies: Rapid NMDA-mediated acquisition of conditioned analgesia

A 30-min exposure to intact biting flies (stable flies) induced an opioid-m ediated analgesia in By-naive male deer mice, whereas exposure to either al tered biting flies whose biting mouthparts were removed or nonbiting house flies had no significant effects. However, mice that were previously expose d to intact stable flies for 30 min exhibited significant analgesia when ex posed 24-168 hr later to stable flies whose biting parts were removed, but not to nonbiting house flies. Administration of the specific N-methyl-D-asp artate (NMDA) antagonist NPC 12626 to By-naive mice before exposure to inta ct flies, although not significantly reducing the analgesic response, block ed the subsequent conditioned analgesia. Naloxone, which blocked the intact biting fly-induced analgesia, did not alter the acquisition of the conditi oned analgesic response to the altered stable flies. This demonstrates an N MDA-mediated acquisition of conditioned analgesia to a natural aversive sti mulus.